Project Summary As a major detoxicating enzyme in human liver, hydroxysteroid sulfotransferase (SULT2A) catalyzes the sulfonation of biologically important endogenous and xenobiotic substrates, including hormones, cholesterol intermediates, pharmaceuticals and procarcinogens. Sulfonated sterol metabolites are emerging as novel candidates for the modulation of nuclear receptor activity in the liver. We previously demonstrated that SULT2A transcription is regulated by lipid-sensing nuclear receptors in human and rodent liver. Recently, hyposulfatemic mice with nullified expression of a renal sulfate transporter were found to develop altered hepatic lipid and cholesterol metabolism along with selectively induced expression of hepatic SULT2A. These results suggest a role for sulfate homeostasis as an under-studied facet of metabolic disease. Our hypothesis is that hepatic SULT2A1 is an environmentally-sensitive determinant of liver X receptor (LXR) signaling. SULT2A1 catalyzes the sulfonation of endogenous sterols, thereby modulating the expression of LXR target genes, including SULT2A1 itself. SULT2A1 transcription is regulated (1) by the xenobiotic-sensing receptor, pregnane X receptor (PXR), through interactions involving hepatocyte nuclear factor 4 (HNF4) and (2) under conditions of sulfate depletion, through the above-described LXR-mediated autoregulatory mechanism. The specific aims of the proposed research are to: (1) Define the roles of HNF4 and PXR in the modulation of human hepatic SULT2A1 transcription by rifampicin, (2) Define the mechanism mediating SULT2A up-regulation in response to hyposulfatemia, (3) Identify endogenous hepatic sterols that represent physiological substrates for SULT2A1, and (4) Determine the abilities of sulfonated sterols, relative to their unsulfonated counterparts, to function as modulators of human LXR activity. Overall, this work has implications for metabolic disturbances in cholesterol and sulfate metabolism in humans, and will provide new insights into the interactive roles of nuclear receptors and endogenous intermediates as modulators of xenobiotic detoxication and metabolism in human liver.